Image courtesy Randy Pench/The Sacramento Bee

The near-catastrophe at the Oroville dam in February 2017 highlights the need and opportunity to manage flood risk through green infrastructure upstream and downstream of urban and rural communities. Our Green Infrastructure and the Oroville Damreport, developed by Sierra CAMP in partnership with the Capital Region Climate Readiness Collaborative, provides more information about the importance of green infrastructure to the Sierra and our urban counterparts.

What happened at Oroville?

The Oroville Dam, located in the Sierra Nevada foothills on the Feather River just a few miles east of the City of Oroville, is the tallest dam in the United States and encloses the second largest reservoir in California, Lake Oroville. The dam was built by the California Department of Water Resources in 1961, and began operating in 1968. Its structure consists of an earthfill embankment, a main concrete spillway, and an earthen emergency spillway. The dam also generates hydroelectricity in an underground power station, the Edward Hyatt Pump-Generating Plant. Water from the lake supplies the California Aqueduct, which flows into the San Joaquin Valley and the Southern California coast.

In February 2017 after multiple high-volume storms, some of which were characterized as atmospheric river events, Lake Oroville was beginning to reach capacity as inflow rates from the Upper Feather River exceeded annual averages and projected rates. Water flows from the Feather River and its tributaries continued to increase from rapid snowmelt and high precipitation in the Sierra Nevada over the course of a week. As a result, water began to overtop the main spillway. Over the course of a few days, the main concrete spillway partially ruptured and began to erode. Dam operators cut back on the water release to avoid further damage, even as Lake Oroville’s water level continued to rise with inflows from another storm. As the lake’s surface approached the emergency level, dam operators were prompted to allow water to flow over the emergency spillway for the first time in history.

As the emergency spillway came into use it also partially failed as the earthen slope began to erode, which compromised the structural integrity of the emergency spillway wall and raised concerns of a catastrophic release.2 A rupture of the emergency spillway could release a 30-ft wall of water — a grave threat to downstream levies and residents. About 188,000 people were evacuated as a precaution.

The erosion and structural damage to the spillway led to the deployment of 800 personnel from the Department of Water Resources (DWR) in emergency response teams to clean up the more than 1.7 million cubic yards of debris and repair the damaged hillside. In March, the DWR estimated emergency response and recovery costs of $4.7 million a day. In April, the estimated repair cost for the spillways was announced, totaling more than $275 million.

What is Green Infrastructure?

Green infrastructure can mitigate the impact of future atmospheric rivers and extreme storms like the February 2017 Oroville event. Green infrastructure manages water through nature-based processes and ecological systems. It can be either natural (a wetland) or man-made (a park), and provides ecological, hydrological, and other benefits to society and ecosystems. Green infrastructure can also reduce and treat storm water both at the source and downstream in cities and streets. This is in contrast to grey infrastructure and conventional water management systems such as wastewater plants, buildings, pipes, drainage paths, or dams.

Source: Green Infrastructure Research group

Green Infrastructure and the Sierra Nevada

Man-made green infrastructure often takes shape in the restoration or creation of vegetation and healthy soils, or other landscape alterations that increase an area’s ability to absorb and store water. These alterations can be made to urban spaces and nearby land as well as upstream of these areas. Much of the Sierra Nevada landscape provides natural green infrastructure in the form of meadows and wetlands. These places can prevent downstream flooding by serving as natural pathways for water: meadows and wetlands act as sponges, absorbing precipitation at greater capacity, and release it slowly into streams later in the summer, and the streams in turn bring the water down into the foothills and valley. However, human-induced changes, such as river cutting, logging practices, overgrazing, development, parking lots, roads and other impervious surfaces have greatly reduced the Sierra Nevada’s natural capacity for storm water mitigation. According to the National Fish and Wildlife Foundation, some 40 to 60 percent (130,000 to 200,000 acres) of Sierra Nevada meadows are in a degraded state.

Investing in green infrastructure upstream benefits both neighboring communities and downstream communities. It can reduce the stress that heavy precipitation places on grey infrastructure systems like the Oroville Dam and downstream cities, and lower the risk and severity of flooding. Green infrastructure investment can take the shape of meadow restoration, which can restore the ability of meadows to absorb sudden influxes of water such as heavy winter precipitation and slow down its release into nearby water flows. This increase in water absorption enables healthy meadows to shift the temporal distribution of streamflow, as they retain water and thus reduce peak flows during winter and spring while increasing water flow during late summer.

source: nature conservancy

Knowledge about the physical impacts of climate change expected in northern California is growing, and the impacts are projected to be significant. Droughts alternating – and in some cases, co-existing – with extreme storm events set the stage for dangerous floods. Wildfires in the Sierra Nevada forests and foothills can increase the speed of runoff and erosion. Oroville serves as a wake-up call to a climate change-fueled hydrological crisis shaped as much by the way the landscape in the watershed above the lake and the dam responded to the extreme storms as it was by the condition of the dam and the spillway. A holistic consideration of the watershed – including the characteristics and the complexity of both the uplands and the lowlands — is essential for effective adaptation to a dynamic climate. Developing more effective ways of storing snow and water in the higher elevations could save money and lives. As such, investment and collaboration around green infrastructure projects, both upstream and downstream, is critical for preserving our natural resources and the health and safety of our communities.